This application claims priority under 35 USC §119 to Korean Patent Application No. 10-2007-0116590, filed on Nov. 15, 2007 in the Korean Intellectual Property Office, the disclosure of which is incorporated herein in its entirety by reference.
1. Field of the Invention
The present invention relates generally to image sensors, and more particularly to an image sensor with adjusted signal gains in the active and black pixels.
2. Background of the Invention
An image sensor converts an optical image into electric signals. Recent developments in computers and telecommunications result in great demand for image sensors with improved performance such as for use in digital cameras, camcorders, personal communication systems (PCSs), gaming devices, security cameras, medical microcameras, and robots.
Metal-oxide semiconductor (MOS) image sensors are easy to drive using various scanning methods. In addition, a signal processor is easily integrated into a MOS image sensor as a single chip such that the MOS image sensor may be miniaturized with reduced manufacturing cost using typical MOS fabrication processes. Furthermore, power consumption of a MOS image sensor is generally low such that the MOS image sensor is amenable for use in devices with limited battery capacity. Accordingly, use of MOS image sensors capable of realizing high resolution has dramatically increased.
A MOS image sensor includes an active pixel region having a plurality of active pixels and an optical black region having a plurality of black pixels. Photoelectric converters of the active pixels generate charge from photoelectric conversion of light and also from thermal energy. In contrast, photoelectric converters of the black pixels generate charge from only thermal energy since light is shielded by a light shield layer formed over the black pixels.
Such charge generated by a black pixel as represented by a respective voltage signal is subtracted from the total charge generated by a corresponding active pixel as represented by a respective voltage signal for determining the amount of charge generated by photoelectric conversion in the active pixel. For example, an auto dark-level compensation circuit (ADLC) performs subtraction on such respective voltage signals to generate a digital image signal corresponding to the amount of charge generated by photoelectric conversion.
The subtraction result is desired to eliminate a dark signal level generated from thermal energy in the active pixel.